It is generally accepted that it is most difficult to treat molten iron with magnesium so that it can be desulfurized or nodularized. This difficulty arises from a variety of physical characteristics which include (a) the typical treatment temperature for molten iron is usually at about 2600.degree.-2800.degree. F. and magnesium is in vapor form at that temperature level; (b) the solubility of magnesium into molten iron is extremely low; magnesium is a very light material and due to its low density tends to float on the molten metal and become oxidized; (d) magnesium oxidizes extremely rapidly when it comes into contact with air; and (e) magnesium is extremely reactive with molten iron and produces considerable pyrotechnic display which may consist of bursts of iron particles resulting from such reactivity.
The prior art has attempted to carry out the magnesium reaction according to principally four methods: the sandwich method, the injection method, and plunging process, and the Fisher or Kuboto processes requiring a pressure type reaction chamber. The sandwich method involves diluting the magnesium by alloying with nickel or silicon so that when the diluted material is brought into contact with the molten iron, which is preferably laid on the bottom of the molten vessel, a reduced magnesium vapor pressure will result and thus retard the tendency to send off magnesium vapors with extreme reactivity. Examples of magnesium alloys include Mg-Ni and Mg-Fe-Si. Unfortunately, these alloys are either expensive or insufficiently heavy so that additional steel cover of particles is necessary to prevent them from floating upwardly in the reaction ladle. The principal difficulty with the sandwich method is that the recovery of magnesium is low at about 30-50% of the magnesium that is added to the process. (Recovery shall mean herein the ratio between the units of a material added to a process and the units of the material appearing in the final metal product plus that combined with impurities).
Although not commercially used, the injection of magnesium powder takes place by the use of an inert vehicle such as nitrogen gas. It is typical for such magnesium powder to carry an oxide coating thereon by the mere nature of the production of the magnesium particles. The recovery of magnesium in the final metal is low (30% recoveries are typical), due to the floating of the powder inhibiting proper reaction and to dilution resulting from the formed oxides.
The plunging process uses a block of pure magnesium coated with layers of suitable refractory or employs a coke body impregnated with pure or high magnesium, each of which are plunged (carried mechanically) into the molten bath of iron. If carried out in a conventional way with the plunging tool introduced from the top of the open ladle and carried close to the bottom of the vessel, the recovery of magnesium will be 30-40%. The plunging process and Fisher or Kuboto processes are disadvantageous because a large mass of magnesium is allowed to react uncontrollably and special apparatus is required to obtain or contain access to the molten metal.
What is needed is a method which permits simple predetermined adjustment of the magnesium additive to achieve a more controlled reaction with molten metal without the need for special or expensive apparatus. The method should employ hydrostatic pressure of the molten metal to contain any magnesium vapor rendering a higher efficiency in graphitizing or desulfurizing of the metal. It is also important to carry out such reaction without diluting the magnesium which affects efficiency of magnesium recovery.